1. Field of the Invention
The present invention relates to a sheet supplying apparatus used with a recording apparatus such as a printer, a copying machine, a facsimile and the like.
2. Related Background Art
In conventional recording apparatuses such as printers, copying machines, facsimiles and the like, a thick sheet such as a post card, an envelope or a special sheet such as a plastic film are used as a sheet, as well as a plain sheet. The sheet is manually supplied or inserted one by one, or the sheets are automatically and successively supplied by means of a sheet supplying apparatus.
FIGS. 44 and 45 show an example of a conventional recording apparatus integrally incorporating a sheet supplying apparatus therein. In FIGS. 44 and 45, the recording apparatus comprises a sheet supplying apparatus 100 for separating and supplying sheets P one by one, and a recording portion 101 for recording an image on the supplied sheet P. The sheet supplying apparatus 100 has a pressure plate 103 shiftable with respect to a base (sheet stacking means) 102, and a sheet stack P rested on the pressure plate 103 is urged against a sheet supply roller 105 and rollers 105 coaxial with the sheet supply roller by means of a pressure spring 104. Further, a separation pawl 107 is located at a position corresponding to one of front corners of the sheet stack P. The separation pawl 107 is disposed at only one corner of the sheet stack.
Tip end of the sheets P stacked on the pressure plate abut against a sheet tip end abutment portion 102a provided at a lower end of the base 101. As shown in FIG. 45, an abutment surface of the sheet tip end abutment portion 102a is inclined with respect a surface of the sheet stack P within an angular range of .beta..degree. to .gamma..degree.. When a sheet having relatively small resiliency such as a thin sheet is used, as shown in FIG. 45, the sheets are supported in an inclined condition in such a manner that one front corner of the sheet stack P is regulated by the separation pawl 107 and the other front edge of the sheet stack (not regulated by the separation pawl) abuts against the sheet tip end abutment portion 102a. When the sheet supply roller 105 is contacted with the sheet stack P and is rotated, a conveying force acts on the sheet stack P.
On the other hand, since one front corner of the sheet stack P is regulated by the separation pawl 107, only an uppermost sheet P overcome the resistance of the separation pawl 107 to ride over the latter, thereby separating the uppermost sheet from the other sheets. The separated sheet is supplied to the recording portion 101.
When a sheet having relatively great resiliency such as an envelope and a post card is used, since the sheet is not flexed in the vicinity of the separation pawl 107 sufficient to be separated by the separating action of the separation pawl 107, it is difficult to separate the uppermost sheet from the other sheets by the separation pawl 107. Thus, regarding the sheet P having relatively great resiliency, the separation pawl 107 (located at one front corner of the sheet stack) is spaced apart from the sheet stack, and the sheet is separated in the following manner. The sheets having relatively great resiliency are prevented from advancing toward a downstream side due to friction between the tip end of the sheet stack and the sheet tip end abutment portion 102a when the tip end of the sheet stack abut against the sheet tip end abutment portion 102a (regulated condition). In order to release such a regulated condition, it is necessary to overcome the resiliency of the sheet to flex the sheet and to provide a conveying force sufficient to feed the sheet P toward the downstream side in opposition to the friction between the tip end of the sheet stack P and the sheet tip end abutment portion 102a.
When the uppermost sheet directly contacted with the sheet supply roller 105 is subjected to the conveying force from the sheet supply roller 105, the conveying force of the sheet supply roller 105 indirectly acts on the other sheets (other than the uppermost sheet) due to friction between the sheets. In consideration of this fact, the inclination angle .beta..degree. to .gamma..degree. (with respect to the surface of the sheet stack P) of the abutment surface of the sheet tip end abutment portion 102a is selected so that only the uppermost sheet P directly subjected to the conveying force of the sheet supply roller 105 is supplied and the other sheets P are regulated (i.e., not supplied). With this arrangement, the sheets having relatively great resiliency can be separated and supplied one by one.
Further, in such a recording apparatus, it is requested that the sheet can be supplied or inserted one by one manually. To satisfy this requirement, the inclination angle of the pressure plate 103 and the inclination angle .beta..degree. to .gamma..degree. (with respect to the surface of the sheet stack P) of the abutment surface of the sheet tip end abutment portion 102a are selected so that the sheet P can be inserted up to the sheet tip end abutment portion 102a without interference and can reach a convey roller (not shown) through the sheet tip end abutment portion 102a.
As mentioned above, the inclination angle .beta..degree. to .gamma..degree. (with respect to the surface of the sheet stack P) of the abutment surface of the sheet tip end abutment portion 102a is selected in a relatively narrow angular range so that both the sheets P having relatively small resiliency and the sheets P having relatively great resiliency can be held, separated and supplied and at the same time the manual sheet insertion is permitted. However, in the above-mentioned conventional sheet supplying apparatus has the following drawbacks:
(1) Since only one front corner of the sheet stack P is regulated by the separation pawl 107 and the other front edge of the sheet stack is supported by the sheet tip end abutment portion 102a, when the sheets P having relatively small resiliency are used, the other front corner of the sheet stack (not regulated by the separation pawl 107) is often protruded toward the downstream side, thereby causing the skew-feed of sheet in the recording portion 101.
(2) When the sheets P are stacked on the pressure plate 103 for a long time, the other front corner of the sheet stack (not regulated by the separation pawl 107) is gradually protruded toward the downstream side and the regulated condition of the front corner of the sheet stack P regulated by the separation pawl 107 becomes unstable. In such a condition, if the sheet supplying operation is performed, poor sheet separation is generated, thereby causing the double-feed of sheets.
(3) During the stacking operation of the sheets P on the pressure plate 103, after the tip end of the sheet stack P abut against the sheet tip end abutment portion 102a, when the sheet stack P is slid laterally until a side edge of the sheet stack is contacted with a side reference surface 102b for positioning the side edge of the sheet stack P, since the tip end of the sheet stack is shifted along the sheet tip end abutment portion 102a, the tip end of the sheet stack is gradually slid down from the sheet tip end abutment portion 102a toward the downstream side. As a result, when the side edge of the sheet stack is contacted with the side reference surface 102b, the front corner of the sheet stack P regulated by the separation pawl 107 is shifted toward the downstream side of the separation pawl 107, with the result that the front corner of the sheet stack is not regulated by the separation pawl 107. In this condition, when the sheet supplying operation is performed, a plurality of sheet P not regulated by the separation pawl 107 are supplied at once.
(4) In order to amend the poor sheet tip end holding ability described in the above items (1) and (2), if the inclination angle .beta..degree. to .gamma..degree. (with respect to the surface of the sheet stack P) of the sheet tip end abutment portion 102a is made smaller, when the sheet is manually supplied one by one without rotating the sheet supply roller, particularly regarding the sheet having the small resiliency, after the tip end of the sheet abuts against the sheet tip end abutment portion 102a, the sheet P cannot be further advanced, thereby making the manual sheet supply difficult.
(5) When the sheet supplying apparatus is used with an ink jet recording apparatus, it is requested that a special sheet having a surface on which special coating agent capable of improving coloring ability and preventing ink-stain to achieve high quality image recording is coated can be used. In this case, while the sheet P is being separated and supplied by the sheet supplying apparatus 100, the coating agent on the surface of the sheet is scraped by the rubbing of the sheet by means of the sheet supply roller 105 and/or the catching of the front corner of the sheet by means of the separation pawl 107. Consequently, the fine particles of the scraped coating agent are deposited on the sheet tip end abutment portion 102a to gradually increase the frictional resistance between the sheet tip end abutment portion 102a and tip end of the sheet, with the result that, as a large number of special sheets P are used, it is difficult to supply the sheet by the sheet supply roller 105.